1. Field of the Invention
The present invention relates to a voltage detecting circuit.
2. Description of the Related Art
In a device using rechargeable batteries, such as a laptop PC, a voltage of each battery must be detected accurately to manage charging/discharging of the batteries that are serially connected. FIG. 18 is a diagram that shows a structure of a common battery voltage detecting circuit (See Japanese Patent Application Laid-Open No. 2002-243771). A battery voltage detecting circuit 100 detects each voltage of four serially-connected batteries, BV1 to BV4. The battery voltage detecting circuit 100 includes an operational amplifier 110, resistors R1 to R4, switches SW0M to SW4M and SW0P to SW3P, and a power source 115 that outputs a reference voltage VREF. To detect a voltage VBV4 of the battery BV4 using the battery voltage detecting circuit 100, the switches SW4M and SW3P are turned ON, and the other switches are kept OFF. In this manner, a voltage VOUT, in proportion to the difference between a voltage V4, a voltage at a positive terminal of the battery BV4, and a voltage V3, a voltage at a negative terminal thereof, is output from the operational amplifier 110 to an AD converter (ADC) 120. The ADC 120 converts the voltage VOUT into a digital value, so as to allow detection of the voltage VBV4 of the battery BV4. A voltage VBV3 of the battery BV3 can be detected in the similar manner, by turning the switches SW3M and SW2P ON and keeping the other switches OFF. The voltage VBV2 of the battery BV2 can be detected by turning the switches SW2M and SW1P ON and keeping the other switches OFF. Also, the voltage VBV1 of the battery BV1 can be detected by turning the switches SW1M and SW0P ON and keeping the other switches OFF.
If the batteries BV1 to BV4 are lithium-ion batteries, each of the voltages VBV1 to VBV4 between both ends of each of the batteries BV1 to BV4 reaches approximately 4.5 volts when the batteries are fully charged. Assuming that each of the voltages VBV1 to VBV4 of the batteries BV1 to BV4 are 5 volts to give extra allowance for design, the batteries BV1 to BV4, connected serially, can generate a total voltage of 20 volts. Therefore, the battery voltage detecting circuit 100 must be tolerable against a high voltage. On the contrary, the control circuit, including the ADC 120, generally utilizes a power source of approximately 3.3 volts. Therefore, the voltage VOUT output from the battery voltage detecting circuit 100 must be 3.3 volts or less.
Assuming that the resistance of the resistors R3 and R4 are R3 and R4, respectively, a gain GAMP of the operational amplifier 110 can be expressed as R4/R3. Therefore, the output VOUT, output to detect the voltage VBV4 of the battery BV4, can be expressed as VOUT=VBV4GAMP+VREF=(V4−V3)R4/R3+VREF. Then, in order to achieve VOUT≦3.3 volts when the VBV4 is 5 volts and VREF is 0.2 volts, the gain GAMP of the operational amplifier 110 must be GAMP≦(VOUT−VREF)/VBV4=(3.3−0.2)/5≈0.6. Based on this calculation, the voltage VOUT, output to the ADC 120, can be brought down to 3.3 volts or less by selecting the resistors R3 and R4 so that the GAMP of the operational amplifier 110 becomes approximately 0.6. However, in this arrangement, the operational amplifier 110 also must be tolerable against a high voltage. This could lead to increase the cost of the battery voltage detecting circuit 100.
In order to use an operational amplifier without the tolerance against a high voltage, the voltage applied to the operational amplifier 110 must be 3.3 volts or less. In other words, to reduce a voltage V+ that is applied to the positive input terminal of the operational amplifier 110 to 3.3 volts or less, the following condition must be met: (V3−VREF)R4/(R3+R4)+VREF≦3.3. If this condition is met, R4/(R3+R4)≦(3.3−VREF)/(V3−VREF)=(3.3−0.2)/(15−0.2)=3.1/14.8≈0.21. This makes the gain GAMP of the operational amplifier 110 to be calculated as GAMP=R4/R3≦0.21/(1−0.21)≈0.26. Therefore, an operational amplifier 110 without the tolerance against a high voltage can be used by selecting the resistors R3 and R4 so to bring the GAMP of the operational amplifier 110 to approximately 0.26. However, because the gain GAMP of the differential amplifier, which is comprised of the operational amplifier 110, is kept small, the voltage VOUT, input to the ADC 120, also becomes low. To detect the battery voltages at high accuracy in this arrangement, the ADC 120 must be highly accurate. This, again, leads to increase the cost.
Furthermore, upon detecting the voltages of the battery BV1 to BV4 with the battery voltage detecting circuit 100, the currents go through the resistors R1 and R3 connected to the input terminals of the operational amplifier 110. Therefore, in order to prevent these currents from discharging the batteries BV1 to BV4, the resistances of the resistors R1 and R3 must be high as a several mega ohms. Furthermore, the resistors R1 to R4 must be less voltage-dependent to allow accurate detection of the voltages of the batteries BV1 to BV4. To manufacture an integrated circuit having resistors with a high resistance and low voltage-dependency, special processes need be implemented. This also leads to increase the cost.